An adolescent female with a medical history significant for absent corpus callosum, polycystic ovarian syndrome, and papillary thyroid cancer status post–total thyroidectomy and I-131 ablation, presented with a 1-year history of submental swelling. She had no associated symptoms. On examination, she had a 6-cm nontender mass in the submentum. Her floor of mouth (FOM) was soft, and there was no cervical lymphadenopathy. Attempted open biopsy at the time of thyroidectomy revealed only adipose tissue. Ultrasonography (US) (Figure, A) revealed a well-circumscribed, avascular, homogeneous, isoechoic lesion, measuring 6.2 × 6.0 × 6.0 cm along the FOM and left submandibular regions. Magnetic resonance imaging (MRI) of the neck revealed the lesion within the FOM, above the mylohyoid muscle. The lesion involved the root of the tongue in the midline and extended to the left, causing significant thinning of the left mylohyoid muscle without extension through it. The lesion demonstrated a fluid signal with T2 hyperintensity (Figure B), T1 hypointensity (Figure, C), and peripheral rim enhancement. Mild restricted diffusion was also noted. The patient underwent transcervical excision of the mass (Figure, D). The mass was encapsulated and positioned above the mylohyoid muscle and extended into the FOM between bilateral genioglossus muscles. The mass was removed entirely via transcervical excision.